Photo taken on April 15, 2001 by Gavin Costello. Used under Creative Commons license.
Sourced from Wikimedia

World Trade Center Memorial by Denise Gould (DOD Photo 060911-F-9471G-006)
DoD doesn’t default to public domain.
Disclaimer as requested by DoD: The appearance of U.S. Department of Defense (DoD) visual information does not imply or constitute DoD endorsement.